Pain after filling

I went to a doctor for cleaning, said i had some cavities that i had to fill.
My teeth never hurt before. I went there the next week, he filled both teeth and said that my upper molar cavity was deep, which is weird cause i never felt anything, he said that if it hurts i might need a root canal. after the filling, I have constant pain that reacts with cold pretty badly, i went to another doctor and told him the story, he ran some tests, put cold ice and my tooth hurt for few mins, and so he said i need a root canal otherwise its going to get worst.. From the x-ray that i have, trying to compare it to online normal teeth xrays, it pretty much looks the same, i dont know how i can spot that i really need a root canal just from the x-ray. Also, my tooth never hurt before, how come that all of a sudden after the filling, it does?
Also, i have to mention that while he was doing the filling, he hit the nerver and it hurt me alot, so he had to put a second numbing syringe, so could it be that he damaged the nerve and its just healing? Its been about a week.
I do not want to do root canal if i dont have to, i dont trust the dentists... for some reason they're not clear with whats wrong. I know that if a decay was bad enough that it would reach the pulp and ruin the nerves and so on, but you would FEEL it by then.
How long should i wait or what should i do at this point?
Thank you.

[Jerome Tsang, DDS]

You can always seek a third opinion with a root canal specialist who will actually determine whether or not you need a root canal and be able to better answer your questions.  

It sounds to me that you had a large cavity in a particular tooth.  Sometimes these teeth don't really hurt or bother you but the trauma from drilling and removing the decay can cause irritation to the nerve.  This sensitivity usually lasts a few days but generally isn't as painful as you described.  The cold test that was performed is a test that we use to determine the functionality of the nerve.  The response is how we figure out whats going on with the nerve.  If its cold but it doesn't hurt, then the nerve is normal.  If it hurts but it goes away rapidly (say about 15 seconds or less), then the nerve is irritated but it has a chance to heal.  If it hurts and lingers, then the nerve is infected and will require a root canal or if it has a weird response such as a sensation of tingling or warm instead of cold. If there is no sensation at all, then the nerve is dead and requires a root canal.  It sounds to me, by the cold test, that the nerve is beyond repair and will require a root canal but seeking a third opinion from a doctor you trust is the most important because you want to feel comfortable with the needed treatment.  But don't go around looking for a doctor who'll tell you what you want to hear either.  Good luck.